How to ID them

I have 24 chicks (8 weeks old) and many look very similar. We have come up with several ways to mark them so we can identify them, but most have drawbacks that caused us to abandon the idea, ie zip ties with numbers written on them--might tighten up accidentally and cause injury to the legs, or tie various colors of cross stitch thread to their legs--they might pull it off and eat it...

Anyway, suppose I was to write on their leg, just a little number, with a sharpie?

Anyone have any other ideas? We just want to ID them so we can get a better idea of which are pullets and which are cockrels. We can only keep one roo, so we have to figure out something soon.